Silicones, particularly the liquid polydimethylsiloxanes generally known as silicone oils, are fluid over a wide range of degrees of polymerization and have excellent interfacial properties due to their different affinities for organic resins. As a result, they have become widely used as additives for improving the moldability, slip or lubricity, release properties, and mechanical properties of thermoplastic resins (refer to Japanese Patent Publication Number Sho 57-170950).
However, these liquid polydimethylsiloxanes are by nature fluid in addition to being difficult to compatibilize with thermoplastic resins, and as a consequence have poor processing and handling properties and are difficult to mix to homogeneity into solid thermoplastic resins. Thus, as discussed below, a number of problems occur when liquid polydimethylsiloxanes are mixed into thermoplastic resins. The following procedure is typically used in order to blend liquid polydimethylsiloxane into thermoplastic resin: the polydimethylsiloxane is first mixed with a small portion of the thermoplastic resin chips, and the resulting mixture and the remaining major portion of the thermoplastic resin chips are then fed to an extruder mixer, where they are melt-mixed. However, the liquid polydimethylsiloxane sticks to the walls of the mixer during preparation of the preliminary mixture, and this prevents its quantitative addition.
Another procedure in use employs melt-mixing by directly introducing the liquid polydimethylsiloxane and unmodified thermoplastic resin chips into an extruder mixer. However, this procedure essentially cannot produce thermoplastic resin in which the liquid polydimethylsiloxane is homogeneously dispersed. Moreover, this procedure requires the use of a special mixer and novel feed devices, such as pumps.
In order to overcome these problems, Japanese Patent Application Laid Open Number Sho 59-215335 proposes a method in which, for example, silica powder and liquid polydimethylsiloxane are mixed to give a moist or wetted powder, and this wetted powder is then melt-mixed into the thermoplastic resin. However, in the case of the silica powder/liquid polydimethylsiloxane mixture prepared in accordance with this method, the liquid polydimethylsiloxane is held in the form of a thin coating layer on the silica powder, and aggregates of autonomous particles are not present. In other words, the structure in which the liquid polydimethylsiloxane is held in a metastable state in the spaces between primary particles in the silica powder, is an unstable structure when duplicated at higher orders. Due to this, the liquid polydimethylsiloxane readily separates from the silica powder during the course of storage for a number of hours. In particular, such a mixture evidences tack or stickiness due to the liquid polydimethylsiloxane present at its surface. As a consequence, this powder will stick to the walls of the machinery, either when initially mixed with thermoplastic resin chips or when introduced into a device such as a direct extruder mixer, and the like. This again causes the problem of a loss of quantitativeness, inter alia, and this method is thus not entirely satisfactory.